The present invention relates to a crystal layer separation method, a laser irradiation method used therefor, and a method of fabricating devices using the same. In particular, the present invention relates to a method of separating a crystal layer from a substrate by irradiating the interface therebetween with a laser beam in a line-shape, and a laser irradiation method used therefor, and a method of fabricating devices using the same.
With respect to a process of fabricating semiconductor light emitting devices by using GaN based compound semiconductors, there has been known a technique of separating a crystal layer made from a GaN based compound formed on a sapphire substrate from the sapphire substrate by irradiating the interface therebetween with a laser beam from the back surface of the sapphire substrate.
For example, Japanese Patent Laid-open No. 2000-101139 has disclosed a technique of forming a GaN layer on a sapphire substrate, and irradiating the interface between the GaN layer and the sapphire substrate with a laser beam from the back surface of the sapphire substrate, to partially decompose GaN forming the GaN layer, thereby separating the GaN layer from the sapphire substrate. This technique, however, teaches only the irradiation density of the laser beam and the laser beam having a wavelength liable to be absorbed in the GaN layer.
To efficiently separate a crystal layer made from a GaN based compound from a sapphire substrate, there has been also proposed a technique of enlarging an area of the laser beam per one shot, and sequentially irradiating the interface between the sapphire substrate and the crystal layer with the laser beam, thereby separating the crystal layer from the sapphire substrate with a high throughput.
By the way, to separate a crystal layer made from a GaN based compound formed on a sapphire substrate therefrom by irradiating the interface between the sapphire substrate and the crystal layer with a laser beam from the back surface of the sapphire substrate, it is important to project a laser beam having an irradiation energy equal to or more than a threshold value required for decomposing the GaN based compound into Ga and N2. On the other hand, at the time of laser irradiation, the decomposition of GaN results in generation of N2 gas. The N2 gas is expanded, to give a shear stress to the GaN layer, thereby tending to cause cracks at a boundary among irradiation regions irradiated with the laser beam. For example, as shown in FIG. 20, if each of the irradiation regions, which are denoted by reference numerals 201 in the figure, has a square shape, a crack 202 may occur at the boundary among the irradiation regions.
In particular, in the case of fabricating devices on a crystal layer made from a GaN based compound having a thickness of several μm or less, the strength of the crystal layer may become insufficient against a shear stress caused by generation of N2 gas, whereby cracks easily occur in the crystal layer. In this case, the cracks occurring in the crystal layer made from a GaN based compound may be propagated to another crystal layer stacked thereon, and at the worst case, devices formed on the crystal layer made from a GaN based compound be broken. Such a problem becomes significant in the case of forming micro-sized devices. If cracks may occur in devices, the devices fail to exhibit specific performances, thereby lowering the yield of devices formed by separating the devices from a substrate by laser ablation.